Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation, or implantable pacemakers which regulate the beating of the heart. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case or a coil electrode of another lead. The electrode typically serves as the cathode (negative pole) and the case or coil typically serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the more proximal electrode typically serving as the anode and the more distal electrode typically serving as the cathode.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (an atrium or a ventricle). A dual-chamber system stimulates and/or senses in at least one atrial chamber and at least one ventricular chamber. Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Recently, there has been the introduction of pacing systems that stimulate multiple sites in the same chamber. These are termed multisite stimulation systems.
When the patient's own intrinsic rhythm fails, pacemakers can deliver pacing pulses to a heart chamber to induce a depolarization of that chamber and this is followed by a mechanical contraction of that chamber. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial depolarizations (detectable as P waves) and intrinsic ventricular depolarizations (detectable as R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart. Pacemakers, as will be described in more detail below, may also deliver pacing pulses to one or more heart chambers to maintain a desired synchrony between the chambers. To this end, the sensing circuits are used to determine whether the pacing of the chamber(s) are effective in causing depolarization at the desired times.
When a pacing pulse is effective in causing depolarization of the heart muscle, it is referred to as “capture” of the heart. Conversely, when a pacing pulse is ineffective in causing depolarization of the heart muscle, it is referred to as “lack of capture”, “loss of capture” or “non-capture” of the heart. These terms should be considered synonyms and will be used interchangeably in this discussion.
The energies of the applied pacing pulses must be above the pacing energy stimulation or “capture threshold” of the respective heart chamber to cause the heart muscle of that chamber to depolarize. More specifically, the capture threshold represents the amount of electrical energy required to alter the permeability of the myocardial cells to thereby initiate cell depolarization. If the energy of the pacing stimulus does not exceed the capture threshold, then the permeability of the myocardial cells will not be altered and thus no depolarization will result. As a result, there will be failure in sustaining the pumping action of the heart. In contrast, if the energy of the pacing stimulus exceeds the capture threshold, then the permeability of the myocardial cells will be altered such that depolarization will result. The pacing energy level is a function of current, voltage and pulse duration (time). Accordingly, the pacing energy level can be adjusted by adjusting one or more of current, voltage and pulse duration.
The capture threshold is not fixed, but rather, may increase and decrease during of the course of a single day, on a daily basis, as well as in response to changes in cardiac disease status. Changes in the capture threshold may be detected by monitoring the efficacy of stimulating pulses at a given energy level. If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy should be increased. In contrast, if capture occurs consistently at a particular stimulation energy level over a relatively large number of successive stimulation cycles, then it is possible that the capture threshold has decreased such that the stimulation energy is being delivered at level higher than necessary to effect capture. This can be checked by lowering the stimulation energy level and monitoring for capture, or loss of thereof, at the new lower energy level.
To reduce current drain on the power supply, it is desirable to automatically adjust the pacemaker such that the amount of stimulation energy delivered to the myocardium is maintained at a level that will reliably capture the heart without wasting power. Such a process can be referred to as “automatic capture verification and threshold search”, but is often referred to by other names. An exemplary proprietary automatic capture verification and threshold search algorithm is referred to as Autocapture™.
While there are certainly variations in how and when an automatic capture verification and threshold search may be performed, they all have a similar goal, which is generally to determine whether a delivered pacing stimulus results in depolarization of the paced myocardial chamber, and, consequently, to adapt the stimulation pulses to a level somewhat above (e.g. a margin above) that which is needed to maintain capture.
An automatic capture verification and threshold search can be performed when a device is implanted, and from time to time thereafter so that pacing stimulation levels are appropriately adjusted as patient conditions change. For example, an automatic capture verification and threshold search algorithm can be performed whenever two consecutive pacing pulses fail to evoke capture, and/or may be performed periodically (e.g., every 8 hours, every 24 hours, etc). The following patents, each of which are incorporated herein by reference, provide details of various exemplary automatic capture verification and threshold search algorithms: U.S. Pat. No. 6,179,622 (Mann et al.) entitled “Method and Apparatus of Determining Atrial Capture Threshold While Avoiding Pacemaker Mediated Tachycardia”; U.S. Pat. No. 7,062,327 (Bradley et al.) entitled “Method and Apparatus for Providing Atrial Autocapture in a Dynamic Atrial Overdrive Pacing System for Use in an Implantable Cardiac Stimulation Device.”
Depending on the pacing mode that is being used, automatic capture verification and threshold search can be performed in the atrium and/or in the ventricles. When performed in the atrium, this process can be referred to more specifically as atrial automatic capture verification and threshold search. Similarly, when performed in the ventricles, this process can be referred to more specifically as ventricular automatic capture verification and threshold search.
In one known automatic capture verification and threshold search technique, the pulse generator applies a succession of primary pacing pulses to the heart at a basic rate. To assess the threshold, the output of the primary pulse is progressively reduced. The output of each successive pair of primary pacing pulses is reduced by a known amount and capture is verified following each pulse. If a primary pulse results in loss of capture, a higher output backup pulse is applied (e.g. about 60-100 milliseconds after the primary pulse which failed to capture the heart tissue) to sustain heart activity. If two consecutive primary pulses at the same output level result in loss of capture, the system identifies that output as being below the capture threshold and then starts to increment the output of the primary pulse. The output of successive primary pacing pulses is then incrementally increased until a primary pacing pulse regains capture. The output of the primary pulse which regains capture is the capture threshold to which a working or safety margin (e.g., between 0.20 and 0.30 Volts) is added to determine the pacing energy.
As mentioned above, pacemakers can be used to maintain a desired synchrony between the chambers. This type of pacing is referred to as cardiac resynchronization therapy (CRT) pacing. CRT pacing (also referred to simply as CRT) seeks to normalize asynchronous contractions associated with congestive heart failure (CHF) by delivering synchronized pacing stimulus to the left ventricle and the right ventricle of the heart, which is referred to as bi-ventricular (BiV) pacing. It is noted that the terms “synchronized” and “synchrony” refer to the left and right ventricles contracting at substantially the same time, or at a selected offset from one another. In contrast, the terms “asynchronous”, “desynchronized” and “desynchrony” refer to the left and right ventricles contracting in a disorganized manner, i.e., not consistently at substantially the same time, or not consistently at a selected offset.
Recent studies have suggested that BiV pacing from two left ventricular (LV) sites can improve clinical outcome in CRT patients, likely due to improved hemodynamic response that can be achieved using dual-site LV pacing, in comparison with conventional BiV pacing. To provide such dual-site LV pacing, and more generally, multi-site LV pacing, leads have been developed that include multiple electrodes for placement in the LV chamber. For example, St. Jude Medical Inc. (headquartered in St. Paul, Minn.) has developed the Quartet™ left ventricular pacing lead, which includes four pacing electrodes on the left ventricular lead—enabling up to 10 pacing configurations.
Pacing at more than one site within the LV chamber is referred to as multi-site left ventricular (MSLV) pacing. Dual-site LV pacing is an example of MSLV pacing. When MSLV pacing is used for CRT, the pacing can be referred to as MSLV type CRT pacing. To receive the benefits of MSLV type CRT pacing, the programmed pacing sequence should occur substantially all of the time (e.g., at least 93% of the time). One challenge associated with MSLV pacing relates to detecting “capture” and “loss of capture” so that pacing parameters can be appropriately adjusted to cause capture without wasting the limited energy available from the implantable system's battery.
To assess whether capture occurred in response to a pacing pulse, a sensing vector is used to monitor for an “evoked response” following the pacing pulse. When appropriate, pacing parameters are adjusted to achieve capture without wasting excessive energy. As explained above, automatic capture verification and threshold search algorithms have been developed to achieve this goal. However, such automatic capture verification and threshold search algorithms have typically been developed assuming that only one pacing pulse is delivered in a same cardiac chamber (e.g., the LV chamber) per cardiac cycle. Accordingly, such algorithms may not effectively achieve this goal where more than one pacing pulse is being delivered within the same cardiac chamber (e.g., within the LV chamber) per cardiac cycle. It is also noted that providing an additional pacing pulse per cardiac cycle (e.g., providing two pacing pulses in the LV chamber, as opposed to one pacing pulse) increases the drain on the battery. Accordingly, this increases the importance associated with not using more energy than necessary to achieve capture.
Another condition which may by detected while performing automatic capture verification and threshold search is fusion. Fusion occurs when a paced evoked response occurs essentially simultaneously with an intrinsic depolarization. The result may be an attenuation of the evoked response signal amplitude to a value that is below an evoked response sensitivity setting. If this happens, fusion which is associated with a myocardial depolarization will not be recognized and will be labeled “loss of capture.” It is desired to minimize this type of undesired fusion.